22 May 2013

Down in the underground, scuds lose eyes but keep genes

When animals live caves full time, their descendents often lose their eyes. It has happened over and over and over and over again, in all different kinds of animals. But how this happens is not obvious. Stephen Jay Gould wrote that some people would use cave fish as an argument that “Lamarck must have been on to something” with his idea that acquired characteristics can be inherited. Well, no, that’s not that case, but it is a good example of how tricky thinking about losses can be.

The latest paper to try to sort out eye loss uses small amphipod crustaceans (Gammarus minus). An advantage of working with this particular species is that some populations live out in the sunshine with us, but several populations have gone down in the underground. In this case, Carlini and colleagues have three separate populations that went into caves, and they have their closest relatives, which are not cave dwellers. Each pair of populations acts as a natural experiment.

The eyes do change with the habitat, as expected. The amphipods that live “above” in springs have eyes with about 40 facets (ommatidia), while the cave dwellers’ eyes have about 5 ommatidia.

Using genetic tests, the team found that the genes for making visual pigments, the opsins, were still intact. They had not turned into non-working genes (“pseudogenes”). The genes for the opsins were extremely similar, and in no way as different as the eyes of these little guys were.

What they did find was that the expression of these genes was dialed way down compared to their surface dwelling relatives:

Carlini and colleagues note that this could be related to the overall reduction of the eye, but they attempted to control for this by scaling expression to the size of the eyes.

Carlini and colleagues suggest that the opsin genes are under some sort of pressure to stay “intact” in this species (contrary to suggestion here that there is an advantage to blindness in caves). But the team doesn’t have a suggestion for what the opsin genes might be needed for, although they suggest it might be a non-visual function.

This doesn’t solve the matter of how the animals are reducing the amount of opsins they make. Presumably there is some mutation in a regulatory gene, perhaps even a gene one specific to the visual system.